How to create intermediate vertices of a humpy surface (interpolation)
@eromod's hint was the solution.


I created a huge ovoid (an icosphere that I compressed on one axis for an additional hump on the other axis),
aligned it carefully manually,
extended the original top surface in top direction,
applied a boolean modifier with "Intersect" operation,
and did a bit afterwork on the mesh.


Please leave a comment if there's something I forgot to do proper preparation for rounded edges around the top surface.